feat(agents): pin explicit model on every agent and workflow call

Agents and workflows previously carried no model field, so every
dispatch inherited the session model — including fable, which is
banned for all plugin agents and workflows by owner decree. Every
dispatch now pins opus or sonnet explicitly.

- opus (low-volume judgment gates whose misses silently poison
  downstream work): architect, bencher, debugger, fieldtester,
  grounding-check, plan-recon, quality-reviewer, spec-skeptic,
  tdd-author
- sonnet (mechanical scope, in-loop or fanned out): docwriter,
  glossary-extractor, implementer, spec-reviewer, synthetic-user,
  tester
- workflows: all 13 agent() call sites pin a model — sonnet
  everywhere except the quality-reviewer gate in implement-loop,
  the loop's last correctness check (spec-reviewer only gates
  task-text correspondence; real-bug finding is the documented
  opus strength)
- docs/agent-template.md: model is now a mandatory frontmatter
  field, with the assignment rule and the fable ban recorded

audit/agents/architect.md

@@ -2,6 +2,7 @@
 name: architect
 description: Read-only architecture reviewer. Checks at cycle close whether the codebase still matches the project's design ledger and CLAUDE.md, identifies drift and technical debt with paths and short justifications, and recommends one direction for the next iteration. Names problems; does NOT propose implementations.
 tools: Read, Glob, Grep, Bash
+model: opus
 ---
 
 # architect

audit/agents/bencher.md

@@ -2,6 +2,7 @@
 name: bencher
 description: Hypothesis-driven performance benchmarker. Designs workloads, runs measurements, interprets results to answer "is X better than Y?" — not "is X fast in absolute terms?". Reports evidence including the limitations of the bench design. Does NOT ship features.
 tools: Read, Write, Edit, Glob, Grep, Bash
+model: opus
 ---
 
 # bencher